As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2007 as well known to the skilled person.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Substrates of aluminium or aluminium alloy in the form of sheet or extrusion, are used to make shaped or formed products. In some of these processes parts of (shaped) aluminium comprising substrates are interconnected. One end of a substrate may be interconnected with the other end, or one substrate may be assembled with one or more other substrates. This is commonly done by brazing. In a brazing process, a brazing filler metal or brazing alloy, or a composition producing a brazing alloy upon heating, is applied to at least one portion of the substrate to be brazed. After the substrate parts are assembled, they are heated until the brazing metal or brazing alloy melts. The melting point of the brazing material is lower than the melting point of the aluminium comprising substrate(s).
Brazing, by definition, employs a filler metal, also referred to as a brazing alloy, having a liquidus above 450° C. and below the solidus of the base metal. Brazing is distinguished from soldering by the melting point of the filler metal: solders melt below 450° C.
Brazing sheet products find wide applications in heat exchangers and other similar equipment. Conventional brazing products have a core of rolled sheet, typically, but not exclusively an aluminium alloy of the AA3000-series, having on at least one surface of the core sheet an aluminium clad layer (also known as an aluminium cladding layer). The aluminium clad layer is made of an AA4000-series alloy comprising silicon in an amount in the range of 2 to 18% by weight, and preferably in the range of 7 to 14% by weight. The aluminium clad layer may be coupled to the core alloy in various ways known in the art, for example by means of roll bonding, cladding spray-forming or semi-continuous or continuous casting processes. These aluminium clad layers have a liquidus temperature typically in the range of 540 to 610° C.
There are various brazing processes in use for the industrial scale manufacturing of heat exchangers.
There is vacuum brazing (“VB”) which is carried out at relatively low atmosphere pressure in the order of about 1.10−5 mbar or less. To obtain the optimum conditions for joining to take place, alloys commonly used for vacuum brazing contain purposive additions of Mg of 1% or more. The Mg destroys the hard oxide film of the filler alloy when it evaporates from the brazing sheet during brazing, and further the evaporated Mg plays the role as getter that removes oxygen and moisture remaining in the brazing furnace.
Another brazing process is controlled atmosphere brazing (“CAB”) which is carried out in a dry no oxygen containing atmosphere, preferably using the inert environment of nitrogen, but for example also argon can be used. To facilitate brazing a non-corrosive brazing flux is applied prior to brazing on the pieces to be joined. This brazing flux removes or at least brakes open during the brazing operation the always present oxide layer to allow the molten filler to come into contact with bare metal to form the joint. The aluminium alloys used for CAB should be free of Mg because any Mg is inhibiting the brazing flux action in removing the oxide layer.
Another brazing process is CAB without using a brazing flux, and this process is in particular being used for joining by means of brazing of surfaces inside a heat exchanger with are very difficult to flux. In European application EP-1430988 it is disclosed that for such a process of CAB without using a brazing flux that the brazing sheet product used contains Mg at least in a layer constituting the brazing sheet other than the filler alloy layer, typically the core alloy contains Mg in a range of 0.05 to 1.0 wt. %. Interposed between the core alloy and the filler alloy and diffusion prevention layer is present, such an a Mg-free AA3003-series aluminium alloy. More specifically, the brazing sheet has an aluminium core alloy clad on one or both sides with Al—Si based filler alloy, and has a cladding thickness “a” (μm) of the filler alloy applied on the inside of a hollow structure, a Mg content X (wt. %) of the core material, and a Mg-content Y (in wt. %) of the filler alloy to satisfy relationships of (X+Y)≦a/60+0.5 and X>Y, with preferred stricter ranges.
European patent no. EP-1306207-B1 discloses another fluxless brazing process in an inert gas atmosphere containing a very low oxygen content of up to 1000 ppm, and preferably up to 500 ppm. Furthermore there is disclosed a brazing sheet product comprising of an aluminium core alloy on one or both sides clad with an Al—Si alloy brazing alloy containing 0.1 to 5% of Mg and 0.01 to 0.5% of Bi as an intermediate layer, and a thin covering material clad onto the Al—Si alloy brazing alloy. It is disclosed that during a brazing operation the brazing material in the intermediate layer is molten as the temperature is elevated during brazing, but oxidation of the surface of the brazing material does not occur because the surface is covered with the thin covering material which remains solid. When the temperature is further elevated, the portions with lower melting points, such as a segregation portion of the thin covering material close to the molten brazing material, are locally molten, and then the brazing material seeps and spreads over the surface of the thin covering material due to volumetric expansion. The surface of the brazing material then becomes an emerging face without an oxidation film, and new intensive oxidation does not proceed due to the inert gas atmosphere. Gradually also the thin covering layer melts also in whole or in part. It is mentioned further, that because no thick oxidation film exists, the molten brazing material wettingly spreads even in the absence of brazing flux and Mg. Sufficient amounts of Mg and Bi within the claimed ranges should be used as they are said to consume oxygen in a proximal atmosphere and prevent oxidation of the surface of the molten brazing and the molten thin covering material.
There is a need for further improved brazing sheet and brazing processes by which the interior side of an assembly has not to be provided with a brazing flux.